Optical apparatus for determining finish on paper



OPTICAL APPAHATUSFOR DETERMINING FINISH 0N PAPER June 16, 1931. c. H.MILLIGAN Filed Jul-y 5, 1929 zlwvenht W m NN N Q h, F N QM MN PatentedJune 16, 1931 UNITED STATES PATENT oar-"E.

CHARLES H. mLIG-Ali', OF HAMILTON, OHIO, ASSIGIN'OR TO THE CHAMPIONCOATED PAIEB' COMPANY, OF HAMILTON, OHIO, CORPORATION OF OHIO OPTICALAPPARATUS FOR G FETISH ON PAPER Application flefl lul y 5, 1929. SerialNo. 876,149.

t is common practice to observe finish on paper by holding itin such aposition that lig t falls upon it at a rather lowangle of incidence andis directl reflected to the eye.

I This method, 'althoug in wide use, is only qualitative. r

A quantitative method which has been proposed consists in allowing abeam of light to fall upon the paper at an angle of about 57 and thenmeasuringthe degree of larization of the light that exists in the reected beam.

s This method does not determine finish directly and, although it isreasonably satisfactory firhlow finish papers, it isnot reliable for gcoated papers.

:0 One object of the present invention is to provide an opticalapparatus for quantitatively determining the finish on paper directly.

Another object is to provide an apparatus which measures accurately thefinish on paper over a wide range of finish.

Briefly, the invention resides in an apparatus comprising means forcausing a beam of light to fall u on a limited length of the paper to betests at avery low angle of incidence, and means for measuring the inrtensity of the illumination of the beam of E1g.. 2is a"diagrammatic planview of the light specularly" reflected from the paper.

One embodiment of the apparatus orming the subject matter of thisinvention is illustrated in the accompanying drawings, in which: 7 p v aFig. 1 is a longitudinal sectional diagrammatic view of theapparatusshowing the paths of the light to be measured and thecomparison beam of light.

- apparatus shiiwing the path of the beam of light to bemeasured,

Fig. 3 ,is adiagrammatic aratus showing the path 0 am of ht Fig; 4: is-afront view of the apparatus, Fig.6 is a front view of the screen. 1 isaqsource of lightifwhich may be for'exlan view of apthe comparison glosspapers such as highly finished optical wedge insertedin' the ample aprojection lamp. '2 is an opaque screen provided with two apertures, 3and 4.

Aperture 4 is a slot, the characteristics of which will be describedlater. The light reflected from the sample, as indicated by the line AA", passes through the lenses 7 and 8 by which it is imaged on the prism9. The

comparison beam of light from the source 1 which passes throughthecircular aperture 3 in the screen 2, follows the course BB. This beampasses through the lens 10 by which it is imaged on the prism 9. 11 isan path of the beam BB between the lens 10 and the rism 9. This wedgecomprises two plates 0 glass with a gelatine film between them.Thegelatine film is relatively transparent at one end and regularlyincreases in opacity to the other end. The optical wedge 11 carries agraduated scale 15. By means of the pointer 16 mounted on the housin ofthe instrument, the position of the we ge is indicated on the graduatedscale 15. 12 is a lens which receives the two beams of light AA A" andBB from the prism 9 and images them upon the eyepiece aperture 13 as twosemi-circles with a common center line. 17 is a transparent coloredscreen preferably green, through which the field is viewed. ,The wholesystem is enclosed in an opaque housing 14, painted a dull black on theinside.

The '0 eration of the instrument is as follows e sample of paper to betested is held by the support 6 and the we e 11 i gelatine film."

When light' falls upon paper, part of a is reflected spe'cularly, partis. difusely reflected, 100

and part is absorbed by the paper. Paper having a high finish is paperwhich reflects considerable light specularly. The difference in theamount of li ht specularly reflected from two papers 0 different finishis greater at lower an les ofincidence than at higher an les. I havefound that as the angle of inci ence of the light decreases, the test ascarried out with the above described apparatus becomes more and moresensitive as a measurement of finish, reaching a maximum sensitivity atabout 8. Thus at an angle of about the instrument fails to adequateldistinguish the finish between pa rs'w ich to the unaided eye are consierably different. However, at an angle of incidence of 8 to 10, theinstrument easily shows difierences which .are not discernible to theunaided eye.

It is essential for the proper sensitivity 7 of the instrument that thelongitudinal dimension CD of the image CDEF on the paper he as small aspossible. This means that in view of the low angle of incident lightused, the aperture 4 in the screen 5 must be as small as possible in thevertical dimension and also that the source of light must not be tooclose to the aperture. An aperture of about, .015 inch in the verticaldimension is about as small as can be employed, for apertures of a sizesmaller than this give rise to frin e effects. With the' light'source,screen an sample, in about the proportion indicated in Fig. 1, forexampli an image about 7 long (dimension D) and 1" wide (dimension CE)is produced on the sample. By introducing a suitable lens in front ofthe screen 2 or by using "direct sunlight as the source of a somewhatlarger aperture may be employed. However, if the image on the sample 1imuch more than A" in the lon tudinal dimension, the sensitivity of theinstrument is decidedly reduced. That is to say, it is pos sible todetect much smaller differences in finish when the image on the paper islong than when it is A" long, other things being equal. When the-imageexceeds an inch in length the instrument is no more sensitive to finishdifferences than the unaided e e.

The ateral dimension of the li ht on the paperdoes not in general haveto e limited as in the case of the longitudinal dimension. Too broad animage introduces optipaper-approximately 1" in lateral ension is verysatisfactory.

At the low angle of incidence employed,

light;

broadl reflected lig t coming from the paper. Pa-

per having asmoother' surface is like to show a bluish shade on thisaccount, an paper having a rougher surface is likely to have a reddishshade. To overcome these minor differences, the green filter 17 isintroduced into the eyepiece as an additional refinement to aid inmatching the fields.

The instrument is more sensitive in determining paper finish than anyother means of which I am aware. It provides a means of quantitativelymeasurin finish on paper, has a broad range and big sensitivity.

The following values obtained with the instrument show its range andsensitivity Scale reading Mimeograph paper 12.5

On 26-0 Dull finish coated pa er 35.5 Supercalendered boo 40.0 Enamelledbook 50.2 High-finish double coated paper 64.8 Microscope slide 86.3

Check readings within .2 can be easily made by the same observer, andwithin .3 by

diflerent observers.

Although I have shown a photometer of the optical wedge type, it will beunderstood that the invention is not limited to this particular kind butthat any form of photometer may be used.-

It will be seen that the invention comprises an apparatus in which abeam of light alls upon the sample of paper to be tested at a low angleof incidence, less than 20 and preferab y 810, to produce an image uponthe paper less than approximately one inch in the longitudinal dimensionand which measures the intensity of the light specularly reflected fromthe paper.

I claim: p

1. Apparatus for determining finish on paper comprising supporting meansfor a sheet ofpaper to be tested, means for directing a beam of lightonto the paper at an angle of incidence less than 20 to illuminate anarea less than one inch in longitudinal dimension, an eyepiece, meansfor imaging the beam 1 of light reflected from the paper onsaideyepiece, a photometer, means for directing a comparison beam of 'ghtthrou h said. photometer and means for imaging 0 comparison beam oflight on said eye 1eoe., 2. Apparatus as claimed in claim 1 wherein themeans directing the beam of light onto the paper is such that the angleof incidence of sai beam of light is approximately 810. 3. Apparatus asclaimed in claim 1 with means for imaging the comparison beam of lightand thereflected beam on the eyepiece as semicircles having a commoncenter line.

. 4, Apparatus as claimed in claim 1 wherein 5 the photometer is-of theoptical wedge type.

5. Apparatus for determining finish on paper comprising an opaquehousing containing means for supporting a sheet of paper in, a plane, asource of light, an opaque screen between the source of light and saidsupporting means, provided with an aperture permitting light from saidsource to fall upon the paper, said aperture being of such size and sopositioned with respect to said source of light and-said paper that thebeam of light passing therethrougli falls upon the paper at an angle ofincidence less than and illuminates an area less than about one inch inlongitudinal dimension, said screen having 20 a second aperturepermitting a comparison beam of light to pass therethrough, an eyepiece,means for imaging said comparison beam of light and the beam of lightreflected from the paper on the eyepiece and a photometer in the path ofsaid comparison beam of light.

6. Apparatus as claimed in claim 5 wherein v the photometer is of theoptical wedge'type.

7. Apparatus as claimed in claim 5'withmeans for imaging the comparisonbeam of light and' the reflected beam of light on the eyepiece assemicircles havin'ga common center line.

8. Apparatus as claimed in claim 5 where- I 35 in the eyepiece containsa colored screen through which the images may be viewed.

9. Apparatus as claimed in claim 5v where-' in the beam of light passingthrough the first mentioned aperture in the screen 1s directed 40 ontothe paper at an angle of incidence of approximately 810.

10. Apparatus for determining finish on paper comprising supportingmeans for, a sheet of paper to be tested, means for direct- 4 ing a-beamof light onto the paper at an angle of incidence less than 20 toilluminate an area less than one inch in longitudinal dimension andmeans for measuring the intensity 1 of the specularly reflected'beam. JIn testimony whereof, I afiix my signature.

CHARLES H. M'ILLIGAN.

